1. Field of the Invention
The present invention relates to a development device and an image forming apparatus incorporating the development device, such as a copier, a printer, a facsimile machine, a plotter, or a multifunction machine capable of at least two of these, and more particularly, to a development device including a developer agitation container separated from a development portion and an air circulation mechanism for circulating developer between the developer agitation container and the development portion
2. Description of the Background Art
Known development devices included in electrophotographic image forming apparatuses employ a configuration in which supplying developer (toner) to a photoreceptor drum functioning as a latent image carrier, on the one hand, and mixing and agitating the developer and the supplied toner on the other are performed simultaneously in a development device housed in a single casing. The development device includes at least two conveyance screws that transport the developer in opposite directions, thus circulating the developer inside the development device.
At present, there are development devices in which an agitation container to agitate the developer is provided separately from the part of the device that actually develops the image (a “development portion”) with the developer that has been sufficiently agitated in the agitation portion being conveyed to the development portion. This system has the advantage that, since the agitation container is separated from the development portion, the capacity of the development portion can be minimized, thus minimizing the proportion of the development portion occupying the area near the photoreceptor drum. In addition, an efficient way to convey the developer from the agitation container is by a pneumatic system to convey the developer by air from the agitation container. As above-described pneumatic system used for the development device, several approaches are proposed.
In a configuration in which the pneumatic method is used as the conveyance mechanism of the developer, matters can be simplified in that a free path at any place connecting the separate path by using a flexible tube and an air pump can be utilized. However, in a state in which the developer is supplied to a casing having an opening in a part, for example, close to a development sleeve in the development portion, the developer and the toner therein is blown outside. Consequently, failure, such as fouling the surrounding components and uncontrolled scattering of toner on the output image may occur. Such scattering of the developer sullies the interior of the image forming apparatus, thereby also degrading image quality.
In particular, this failure occurs most readily when the development device is activated. This is because any developer remaining in the tube functioning as the transport path after a previous developing process is packed together under its own weight, and bulk density of the developer is increased, clogging the tube. The clogging causes a delay until it is dissolved and the bulk density of the developer is decreased so that the developer can be transported through the tube and the conveyance of the developer is started.
During the delay, the developer in the development portion is circulated by the conveyance screws, and then is moved back to the agitation container through a collection path connected to the development portion. Consequently, the amount of the developer in the development portion is decreased, and in the worst case, the developer may be completely gone from the development portion. In this case, the transported air is blown to a space and the opening that has hitherto been blocked by the developer, and the developer and the toner in the development portion becomes easily blown out through the opening. In some cases, the amount of the scattering developer is dozens of times the amount thereof when the opening is blocked by the developer.
In addition, the delay until the clog is dissolved and the bulk density is decreased to a state in which the developer can be transported, by the air varies depending on a concentration of toner in the developer as well as how long the developer has been left sitting and in what state. Consequently, more reliable prevention of the depletion of the developer in the development portion is desired.